It has long been known that television techniques -- particularly with the advent of video tape recording -- can be adapted to a number of types of display other than broadcast television program material. In particular, because of the ease of editing, the wide variety of equipment that is available, the split-screen and superimposition techniques and other effects that can be easily accommodated using television equipment, the adaptation of television techniques to such diverse fields as billboard advertising, score boards and replay boards for sports stadia, screens for showing closed circuit sports and entertainment program material, and the like, is entirely practicable. However, the known devices for presenting the flat screen video display of the program material once it has been photographed, recorded, edited or devised using television techniques, are for the most part inadequate.
Many of the known, large scale video display screens and other apparatuses employ television projection apparatus, which by its very nature is extremely sensitive -- being very carefully optically adjusted -- and which also suffer from a lack of brightness as the screen size is enlarged to any reasonable size. Other devices employ a plurality of liquid crystals or gas discharge devices, or the like, but require enormous amounts of power to drive them and extremely complicated addressing and driving systems. For example, U.S. Pat. No. 3,329,831 in the name of Fuji Neon K.K. shows the production of video pictures using discreet elements, where vertical gating signals and horizontal writing signals are imposed upon a grid. However, that system has only limited gray scale capabilities. Another patent of interest in the prior art is Sobel U.S. Pat. No. 3,714,374 dated Jan. 30, 1973. That patent comtemplates the use of different light-display elements in a flat panel image display including liquid crystals. Sobel, for about the first time, recognizes the requirement for an energy storage device such as a capacitor, either inherent or supplemental, at each light-display element. However, Sobel also requires the use of breakdown-type switches such as ovons; and because of the power requirements, is limited in size.
Another problem that has occurred in the prior art devices is any reasonable gray scale capabilities, coupled with the ability to cluster light-display elements in such a way as to use colour television technology such as the three-colour technology which is standard in North America. All of this must, of course, be capable of being accomplished with reasonable power consumption. In order to accomplish this, it has so far been seen to be necessary to use incandescent lamps together with memory and driver circuit means for each lamp in the array. However, because incandescent lamps do not have energy storage capabilities, the usual approach has been to provide energy storage outside of the drivers and to use complicated synchronization to bring the video information and the power to each point in an array at the same time.
In contradistinction thereto, the present invention comtemplates the provision of memory/driver circuits where both the memory capabilities for the video information for each displayelement in the array and the driver circuits for providing the necessary power to the lamp at that point are included in the same circuit, and where the memory in fact gates the power switching device. In this way, gray scale capabilities are easily provided.
It must be mentioned that, in respect of gray scale capabilities using incandescent lamps, the turn-on and turn-off times of such lamps are sufficiently fast that the lamp comes to its required brilliance very quickly and subsides therefrom sufficiently quickly that no "ghosting" appears, as discussed in greater detail hereafter. The gray scale capability is accomplished by controlling the period of time for which the lamp has been illuminated; the bright end of the gray scale being accomplished by having the lamp continuously or nearly continuously illuminated, and the dark end of the gray scale being accomplished by having the lamp not illuminated or illuminated only for a very short period of time. It has been discovered that, if discreet levels of gray scale information are to be utilized as discussed in greater detail hereafter, sixteen such levels are sufficient -- and, of course, sixteen levels of information can be easily accomplished using four-line, sixteen-bit binary counting. Gray scale capabilities can also be provided using either phase or pulse width modulation of video signals; and it has been determined that such modulation over a period of 1/60 sec. provides ample control with the persistence of the image being retained sufficiently long by the eye of the viewer as to be satisfactory.
In providing the memory and driver circuits of the present invention -- for the sake of simplicity, sometimes referred to as memory/driver circuits -- this invention contemplates the use of either AC or DC power sources from which each lamp in an array may be powered. By so doing, this invention contemplates the use of either positive-going or negative-going video information, as required; and contemplates the use of such solid state switching devices as power transistors or SCR's. Also, the present invention is such that various sampling times can be accommodated, depending upon the resolution that is required. Thus, an acceptable television picture can be obtained using an array of not more than 25 .times. 30 lamps, where the resolution of a standard 525-line picture is quite coarse; and on the other hand, this invention also contemplates billboard-size video display apparatuses having full colour capabilities and using in excess of 20,000 incandescent lamps. The resolution in such displays is quite fine -- being nearly dot-for-dot in comparison with an ordinary North American colour television screen. Thus, the sampling time for each element in a total array must be such as to accommodate all of the elements which are to display video information during each cycle of operation of the apparatus.
To accomplish the above, this invention provides a memory/driver circuit for each lamp of a plurality of incandescent lamps used in a flat screen video display apparatus, where each of the lamps is individually addressable in an X-Y matrix, and where video information for each lamp is derived from a television signal and displayed by illuminating the lamp for a controlled period of time so as to give visual gray scale effects according to the length of time of illumination of the lamp, where the lamp is connected in series with a source of power, consisting of means for detecting and storing a video signal, and comparator means to drive a solid state switching means which controls the period of time for which the lamp shall be illuminated. The means for detecting and storing a video signal are such that the video signal being detected and stored is that which is intended for display at that lamp at the next time that the lamp may be illuminated; in other words, if it is intended that the lamp shall be very bright, a suitable video signal is detected and stored, and likewise if it is determined that the lamp shall not be bright -- i.e. black -- another suitable video signal is detected and stored. The means for detecting and storing a video signal may be continuously connected to the source of video signals, but are operative only upon the coincidence of horizontal and vertical (or X and Y) enabling signals which turn on the remainder of the circuit. The comparator has inputs from the means for detecting and storing video signals and also from reference signal means, and is adapted to give an output signal when the values of the video signal and the reference signal are coincident. The timing of the coincidence of the reference and video signals in each cycle of operation of the memory/driver circuit is, of course, a function of the value of the video signal which was fed to the means for detecting and storing the video signal at the beginning of that cycle of operation. Thus, the solid state switching means is connected to the output of the comparator and in series with the lamp and the source of power so that when the solid state switching means becomes conductive, the lamp is illuminated. The period of illumination of the lamp is thereby controlled.
Where the video signal is an analogue signal, means are provided to sample that signal for a sufficient length of time that all of the lamps to be illuminated during a cycle of operation of the video display apparatus are sequentially provided with the video information. Where the solid state switching device is an SCR and the source of power is an alternating current source such that each pulse of power has an initial slope which is always in the same sense with respect to zero voltage, all of the lamps which are to display video information during a given cycle of operation of the apparatus -- or at least their respective memory/driver circuits -- are made operative at the same time. Where the source of power is a direct current, each individual memory/driver circuit becomes operable when it is addressed with the video information intended for it.
Where the video information intended for any given lamp is a discreet multi-bit video signal level, suitable encoding means must be provided. Also, countup or countdown -- usually countdown -- circuits are provided so that the discreet video signal level information can be read into a decade counter and released from the decade counter upon the coincidence or O,O signal therefrom when the digital count and the discreet video signal level are coincident.